Method for the elimination of mercury from a heavy hydrocarbon-containing feedstock upstream of a fractionation unit

ABSTRACT

Process for the elimination of mercury contained in a heavy hydrocarbon-containing feedstock upstream of a main fractionation unit, a process in which:
         a) the non-elemental mercury contained in the compounds of said feedstock is transformed to elemental mercury,   b) a separation of the feedstock obtained in stage a) is carried out in a separation unit, that consists of producing a liquid effluent and a gaseous effluent comprising elemental mercury;   c) the gaseous effluent originating from stage b) comprising the elemental mercury is brought into contact with a mercury capture material contained in a unit for the capture of mercury, in order to produce an effluent that is at least partially de-mercurized.

TECHNICAL FIELD

The present invention relates to a process for the elimination of heavy metals, and more particularly mercury, that are present in a liquid or gaseous feedstock.

STATE OF THE ART

Mercury is a metallic contaminant that is found in gaseous or liquid hydrocarbons produced in many regions of the world, such as the Niger Delta, South America or North Africa.

The elimination of mercury from hydrocarbon cuts is desirable in an industrial context for several reasons:

-   -   for reasons of the safety of operators: elemental mercury is         volatile and presents serious risks of neurotoxicity via         inhalation, while its organic forms present similar risks via         skin contact;     -   for reasons of preventing de-activation of the heterogeneous         catalysts serving to upgrade these liquid hydrocarbon cuts:         mercury amalgamates very readily with the noble metals such as         platinum or palladium used for various catalytic operations, and         in particular the selective hydrogenation of the olefins         produced by steam cracking or catalytic cracking of liquid         hydrocarbons.

Industrially, the elimination of heavy metals, in particular mercury, from the liquid or gaseous hydrocarbon cuts is carried out by circulating them through beds of capture material. By capture material is meant in the present invention any type of solid in bulk or supported form containing within it or on its surface an active element capable of reacting irreversibly with an impurity such as mercury contained in the feedstock to be purified. The elimination of mercury from the liquid or gaseous hydrocarbon-containing cuts is generally carried out by circulating said feedstock to be treated through beds of capture materials containing an active phase capable of reacting with the mercury. It is in particular known to a person skilled in the art that mercury capture can be carried out easily by reacting the latter with an active phase based on sulphur or a sulphur-containing compound, in particular metallic sulphides, the mercury then forming with the sulphur the chemical species HgS called cinnabar or metacinnabarite. These different chemical reactions are generally implemented in a process by using contact of the feedstock to be treated with a capture material that is either bulk in which in particular particles of the active phase can be bonded together via binders, or supported in which the active phase is dispersed within or on the surface of a porous solid support.

However, it is not possible to carry out such a purification operation directly on crude oil cuts or gas condensates for several reasons. The first is that the porosity of these capture materials would very quickly become clogged by the heavy compounds present in said feedstock, which would be deposited on the surface of the materials. Moreover, these crude oil cuts or gas condensates contain mercury in different forms. In fact, unlike the gas phases, they contain not only elemental mercury but also mercury in complexed or ionic and organic form. Now, these complexed or ionic and organic compounds of mercury are called refractory, as they are stable under normal operating conditions and are not reactive with the capture materials of heavy metals. It therefore appears to be necessary to convert the refractory mercury compounds to elemental mercury.

Numerous means have been developed in order to convert the refractory forms of mercury to elemental mercury (also called mercury in atomic form Hg⁰). For example, U.S. Pat. No. 4,911,825 discloses a process for the transformation of the refractive species of mercury from the feedstock to elemental mercury in the presence of a catalyst and under high hydrogen pressure and at a high temperature.

U.S. Pat. No. 5,384,040 discloses a process for the elimination of the mercury from a hydrocarbon-containing feedstock comprising a stage of transformation of the mercury contained in the compounds of the feedstock to elemental mercury, the transformation stage being carried out between 120 and 400° C. and under pressure of 0.1 to 6.0 MPa. Preferably, the transformation stage is carried out in the presence of a catalyst comprising at least one metal M selected from the group formed by iron, nickel, cobalt, molybdenum, tungsten and palladium. Alternatively, the transformation stage can be carried out in the absence of a catalyst.

In the latter case, the temperature must be set at 180° C. as a minimum. In fact, in the article by Masatoshi Yamada et al. entitled “Mercury removal from natural gas condensate” in the journal Studies in Surface Science and Catalysis, volume 92, pages 433-436, 1995, it is shown that the conversion of diethyl mercury starts at 180° C. and reaches 100% conversion at 240° C. At the same time, it is shown that it is possible to reduce the transformation temperature in the presence of a catalyst. In fact, the conversion of the refractory species of mercury starts at 130° C. and reaches over 90% from 200° C. However, the problem with the use of a catalyst, apart from its cost, is that there is a tendency to promote the cracking of molecules and therefore the formation of coke. Furthermore, in the case of highly clogging feedstocks such as crude oil, a very rapid de-activation of the porous catalyst is noted, due to the deposition of heavy compounds such as asphaltenes, within the pores of said catalyst. Such a process is thus more suitable for the treatment of hydrocarbons originating from a first fractionation.

The Applicant discovered, surprisingly, that it is possible to eliminate heavy metals, and more particularly mercury, contained in a gaseous or liquid feedstock, and more particularly a crude oil feedstock, by carrying out upstream of the main fractionation unit, a stage of heating said feedstock at a target temperature and during a residence time sufficient to allow the transformation of the refractory spaces containing heavy metals, present in different forms, to metals in the atomic (or elemental) form, even in the absence of a catalyst or hydrogen, and by carrying out upstream of the main fractionation unit, a stage of capture of the heavy metals, and more particularly mercury. In fact, although the crude oil feedstocks comprise a very great diversity of molecules, bringing said feedstock up to a temperature during a sufficient residence time upstream of the main fractionation unit makes it possible to convert the majority of the refractory compounds to metallic compounds (also called elemental compounds) that can be captured by a single capture material.

SUBJECTS OF THE INVENTION

The present invention relates to a process for the elimination of mercury contained in a heavy hydrocarbon-containing feedstock upstream of a main fractionation unit, in which process:

a) the non-elemental mercury contained in the compounds of said feedstock is transformed to elemental mercury, said stage being carried out in a conversion unit at a target temperature during a fixed residence time and adapted to said target temperature so that at least 90% by weight of non-elemental mercury contained in the compounds of said feedstock are converted to elemental mercury, said stage of transformation being carried out in the absence of hydrogen and in the absence of a catalyst, it being understood that:

-   -   when the target temperature of said feedstock is comprised         between 150° C. and 175° C., the residence time of said         feedstock in the conversion unit is comprised between 150 and         2700 minutes; and/or     -   when the target temperature of said feedstock is greater than         175° C. and less than or equal to 250° C., the residence time of         said feedstock in the conversion unit is comprised between 100         and 900 minutes; and/or     -   when the target temperature of said feedstock is greater than         250° C. and less than or equal to 400° C., the residence time of         said feedstock in the conversion unit is comprised between 5 and         70 minutes; and/or     -   when the target temperature of said feedstock is greater than         400° C., the residence time of said feedstock in the conversion         unit is comprised between 1 and 10 minutes;         b) a separation of the feedstock obtained in stage a) is carried         out in a separation unit, in order to produce a liquid effluent         and a gaseous effluent comprising elemental mercury;         c) the gaseous effluent originating from stage b) comprising the         elemental mercury is brought into contact with a mercury capture         material contained in a unit for the capture of mercury, in         order to produce an effluent that is at least partially         de-mercurized.

Preferably, in stage b) a separation of the feedstock obtained in stage a) is carried out in a separation unit that consists of producing only a liquid effluent and a gaseous effluent comprising elemental mercury.

In an embodiment according to the invention, the process comprises a stage d) in which the liquid effluent obtained in stage b) is fractionated in a main fractionation unit.

Advantageously, the reduction in the total content by weight of mercury of said feedstock taken before stage a) and after stage c) is at least 90%.

According to the invention, stages a) and b) are carried out separately or simultaneously.

In an embodiment according to the invention, the separation unit of stage b) is a distillation column.

In another embodiment according to the invention, the separation unit of stage b) is a stripping column.

Advantageously, in the stripping column a carrier gas circulates in counter-current with said hydrocarbon-containing feedstock, said carrier gas at least partially originating from a liquid or gaseous fraction of the main fractionation unit.

Preferably, when the carrier gas at least partially originates from a liquid fraction of the main fractionation unit, said liquid fraction is transformed to a gaseous fraction by means of a heat exchanger.

Preferably, the at least partially de-mercurized effluent obtained in stage c) is fractionated in a main fractionation unit.

Advantageously, said hydrocarbon-containing feedstock comprises between 1 and 10 mg of mercury per kg of feedstock, preferentially 1 to 1200 μg/kg, more preferentially 10 to 500 μg/kg.

Preferably, during stage c), said feedstock is brought into contact with a bulk or supported mercury capture material comprising a phase containing at least one metallic sulphide based on a metal M selected from the group constituted by copper, chromium, manganese, iron, cobalt and nickel.

Alternatively, during stage c), said feedstock is brought into contact with a bulk or supported capture material comprising a phase containing at least sulphur in elemental form.

Advantageously, the heavy hydrocarbon-containing feedstock is a crude oil feedstock.

DESCRIPTION OF THE FIGURES

FIG. 1 diagrammatically shows a conventional fractionation process for a heavy hydrocarbon-containing feedstock, and more particularly a crude oil feedstock.

FIG. 2 diagrammatically shows an embodiment of the process according to the invention in which the separation unit 5000 is a stripping column situated upstream of a fractionation unit 3000.

FIG. 3 diagrammatically shows a second embodiment of the process according to the invention in which the separation unit 5000 is a distillation column situated upstream of a fractionation unit 3000.

DETAILED DESCRIPTION OF THE INVENTION

For better understanding of the invention, the description given hereinafter by way of an example application relates to a process for the elimination of heavy metals, and more particularly mercury, in a heavy hydrocarbon-containing feedstock, and more particularly of crude oil. Of course, the process according to the invention can be used for the elimination of other heavy metals, such as arsenic, lead, vanadium and cadmium, contained in a heavy hydrocarbon-containing feedstock.

By heavy hydrocarbon-containing feedstock is meant, within the meaning of the present invention, a feedstock having a density at 15° C. greater than 750 kg/m³, composed essentially of hydrocarbons, but also containing other chemical compounds which, apart from the carbon and hydrogen atoms, have heteroatoms, such as oxygen, nitrogen, sulphur and heavy metals such as mercury, arsenic, lead, vanadium or cadmium. More particularly, said hydrocarbon-containing feedstock comprises between 1 and 10 mg of mercury per kg of feedstock, preferentially 1 to 1200 μg/kg, more preferentially 10 to 500 μg/kg.

By non-elemental mercury is meant any form of mercury other than in the elemental (or atomic) form, i.e. in the organic molecular form, and/or in the ionic form, and/or in complexed forms.

The description of FIG. 1 relates to a conventional process for the elimination of heavy metals contained in a crude oil feedstock; the description of FIGS. 2 and 3 relates to a process for the elimination of heavy metals according to the invention. FIGS. 2 and 3 repeat certain elements of FIG. 1; the references in FIGS. 2 and 3 that are identical to those in FIG. 1 denote the same elements.

Process According to the Prior Art (FIG. 1)

FIG. 1 diagrammatically shows the first treatments undergone by the crude oil with a view to its initial fractionation, generally carried out by atmospheric distillation according to the prior art. In the interests of clarity, the equipment provided (pumps, valves, heat exchangers etc) is not necessarily shown.

A heavy hydrocarbon-containing feedstock, and more particularly a crude oil feedstock, is sent via the pipe 100 into a desalting unit 1000, consisting generally of washing with water. The main function of this stage is to eliminate the majority of the soluble inorganic species contained in the feedstock. The desalted feedstock is then sent via the pipe 101 into a pre-heating unit 2000. The purpose of this stage of pre-heating the desalted feedstock is to bring said feedstock to a temperature close to the temperature of the bottom of the fractionation unit 3000 situated downstream. The pre-heating temperature is generally comprised between 200 and 400° C., and depends on the number of distillation columns used in the main fractionation unit 3000. The pre-heated feedstock is then sent via the pipe 104 to the main fractionation unit 3000.

The main fractionation unit 3000 can comprise one or more distillation columns (in FIG. 1, a single distillation column is shown). The main fractionation unit makes it possible to produce different hydrocarbon cuts depending on their molecular weights and more particularly depending on their difference in volatility. For example, the fractionation of the feedstock by atmospheric distillation associated with the distillation columns of the main fractionation unit allows the separation of the feedstock into different cuts, from the lightest to the heaviest, and more particularly into fuel gases (C1, C2), propane (C3), butane (C4), light gasoline (C5 to C6), heavy gasoline (C7 to C10), kerosene (C10 to C13), gas-oil (C13 to C20/25), or also atmospheric residue (C20/C25+).

At the outlet of the main fractionation unit 3000, the top effluent of the main fractionation unit generally contains hydrocarbon-containing compounds of which 90% of said compounds have a boiling point less than 200° C. at atmospheric pressure (1.01325×10⁵ Pa). The top effluent is sent via the pipe 400 to a secondary fractionation unit 7000 comprising one or more fractionation columns, allowing the production of different hydrocarbon cuts. Generally, at the outlet of the secondary fractionation unit 7000 various hydrocarbon-containing compounds can be distinguished, such as:

-   -   fuel gases evacuated through the pipe 401 comprising a majority         of hydrocarbon-containing species with one or two carbon atoms         (C1/C2) as well as the purification effluents, such as H₂ or         H₂S. In the interests of clarity, a single flow of fuel gases         has been shown in FIG. 1, but this number can vary in an         industrial site according to the choice of the operator;     -   liquid petroleum gas (LPG) evacuated through the pipe 402         comprising a majority of hydrocarbon species with three or four         carbon atoms (C3/C4);     -   the naphtha cuts evacuated through the pipe 403 comprising a         majority of the hydrocarbon compounds with 5 or more carbon         atoms (C5+), the upper limit of the number of carbon atoms         depending on the choice of the cut point utilized at the top of         the main fractionation unit 3000. Furthermore, according to the         fractionation layout selected by the operator, there may be         several naphtha cuts (not shown in the Figure), for example a         heavy naphtha cut and a light naphtha cut.

The hydrocarbon-containing cuts evacuated via the pipes 401, 402 and 403 are generally each treated by a unit for the capture of heavy metals, and more particularly the capture of mercury in elemental form. As shown in FIG. 1, the capture units 8001, 8002, 8003 are generally placed downstream of the main fractionation unit, in the direction of circulation of the feedstock, for each of the hydrocarbon-containing cuts circulating in the pipes 401, 402 and 403. The capture units 8001, 8002 and 8003 each comprise a mercury capture material in the form of a fixed bed. The mercury capture materials can be all those known to a person skilled in the art for the capture of elemental mercury. The de-mercurized hydrocarbon-containing cuts are evacuated through the pipes 411, 412 and 413 respectively. Therefore, because of the presence of a multiplicity of hydrocarbon-containing cuts in such a process, the number of fixed beds comprising said capture materials becomes significant (a capture material is associated with each cut of hydrocarbon-containing compounds originating from the secondary fractionation unit 7000), especially as the number of capture units can be doubled in order to be able to regenerate the capture materials without interrupting the operation of the unit.

Furthermore, in this process layout, different types of capture material must be used in order to treat, on the one hand, the gaseous flows, for example evacuated through the pipe 401, and on the other hand the liquid flows, for example evacuated through the pipe 403, but also the flows that may contain hydrogen, such as certain fuel gases, requiring specifically adapted capture materials.

The Applicant discovered, surprisingly, that it is possible to eliminate the mercury contained in the compounds of a hydrocarbon-containing feedstock, and more particularly in a crude oil feedstock, upstream of a main fractionation unit, by carrying out a stage of transformation of the non-elemental mercury contained in the compounds of said feedstock to elemental mercury, and by carrying out a stage of capturing the elemental mercury upstream of the main fractionation unit of said feedstock, by means of a heat treatment of said feedstock at a target temperature and during a residence time sufficient to allow the transformation of non-elemental mercury contained in the compounds of said feedstock to elemental mercury, without the use of any catalytic or hydrogen treatment. The process according to the invention only requires a single unit for the capture of elemental mercury and therefore a single capture material.

In fact, although the crude oil feedstocks comprise a very great diversity of hydrocarbon-containing molecules, bringing said feedstock up to a temperature during a sufficient residence time upstream of the main fractionation unit makes it possible to convert the non-elemental mercury contained in the compounds of said feedstock to elemental mercury, it then being possible to capture the latter with a single capture material.

The process according to the invention comprises at least:

a) the transformation of the non-elemental mercury contained in the compounds of a hydrocarbon-containing feedstock, and more particularly in a crude oil feedstock, to elemental mercury; b) the separation of said feedstock into a liquid effluent comprising a hydrocarbon-containing feedstock with a low mercury content, and a gaseous effluent comprising the most volatile compounds and mercury in elemental form; c) capture of the mercury in elemental form contained in the gaseous effluent obtained in stage b) in a unit for the capture of mercury comprising a capture material.

According to the invention, stages a) and b) can be carried out separately or simultaneously.

According to the invention, the separation stage can be carried out by means of a separation unit selected from a stripping column with a carrier gas (c.f. FIG. 2) or a distillation column (FIG. 3).

Process According to the Invention (FIG. 2)

With reference to FIG. 2, diagrammatically showing an embodiment of the process according to the invention, a hydrocarbon-containing feedstock, and more particularly a heavy hydrocarbon-containing feedstock, is sent via the pipe 100 into a desalting unit 1000. The desalted feedstock is then sent via the pipe 101 into a unit 900 for the conversion of the non-elemental mercury contained in the compounds of said feedstock to elemental mercury. Within the context of the present invention, the conversion unit 900 corresponds:

-   -   either to the heating unit 2000, such as a drum, and optionally         with a pipe 102 or a set of pipes intended for the transport of         said feedstock to the separation unit 5000. In this embodiment,         stages a) and b) of the process according to the invention are         carried out separately, i.e. the transformation of the mercury         to elemental mercury is carried out upstream of the separation         unit 5000;     -   or to the heating unit 2000, such as a drum, and optionally with         a pipe or a set of pipes intended for the transport of said         feedstock to the separation unit 5000 and to the separation unit         5000. In this embodiment, stages a) and b) of the process         according to the invention are carried out simultaneously, i.e.         the transformation of the mercury to elemental mercury is         carried out both during the transport of said feedstock to the         separation unit 5000 and during the stage of separation of said         feedstock in the separation unit 5000.

When the conversion unit 900 comprises a drum, said drum advantageously comprises a double wall covering the drum in which a heat transfer fluid circulates in order to maintain the temperature of said feedstock at the target temperature up to the separation unit 5000, and/or advantageously comprises a resistive heater inserted directly inside said drum.

When the conversion unit 900 comprises a pipe or a set of pipes, the pipe or set of pipes advantageously comprise a double jacket in which a heat transfer fluid circulates in order to maintain the temperature of said feedstock at the target temperature up to the separation unit 5000.

a) Stage of Transformation of the Non-Elemental Mercury Contained in the Compounds of the Heavy Hydrocarbon-Containing Feedstock to Elemental Mercury

Stage a) of transformation (or conversion) of the non-elemental mercury contained in the compounds of the heavy hydrocarbon-containing feedstock to elemental mercury is essential according to the invention. In fact, regardless of the nature and/or the origin of the heavy hydrocarbon-containing feedstock, the latter can comprise heavy metals, and in particular mercury, in different forms. For example, mercury may be found in the form of elemental or atomic mercury (also called Hg⁰) and/or in the organic molecular form, and/or in the ionic form, for example in the form of Hg²⁺ and complexes thereof.

According to the invention, the transformation of the non-elemental mercury contained in the compounds of the hydrocarbon-containing feedstock to elemental mercury is carried out via a conversion unit 900. Implementation of this stage consists of transforming the non-elemental mercury contained in the compounds of the heavy hydrocarbon-containing feedstock to elemental mercury.

Therefore, according to the invention, the process for the transformation of the non-elemental mercury contained in the compounds of said feedstock to elemental mercury comprises passing said feedstock, at a temperature determined by a person skilled in the art, into a conversion unit 900 during a residence time fixed so that at least 90% by weight, preferably at least 95% by weight, and even more preferentially at least 99% by weight of the non-elemental mercury contained in the compounds of said feedstock are converted to elemental mercury, even in the absence of a catalyst.

Therefore, depending on the temperature of the feedstock, the residence time necessary in order to carry out the transformation of the non-elemental mercury contained in the compounds of said feedstock to elemental mercury corresponds to the following equation (1):

$\begin{matrix} {{\ln\left( \frac{{- \ln}\frac{C_{s}}{C_{0}}}{t} \right)} = {{\ln \left( k_{0} \right)} - \frac{E_{a}}{RT}}} & (1) \end{matrix}$

in which: C_(s) corresponds to the concentration of mercury (apart from elemental mercury) contained in the compounds of said feedstock at the inlet of the conversion unit 900 (in mol·L⁻¹); C_(o) corresponds to the concentration of mercury (apart from elemental mercury) contained in the compounds of said feedstock at the outlet of the conversion unit 900 (in mol·L⁻¹); t corresponds to the residence time (in seconds); k₀ corresponds to the constant of the rate of transformation of non-elemental mercury to elemental mercury (in seconds⁻¹); E_(a) corresponds to the activation energy of the reaction for the transformation of non-elemental mercury to elemental mercury (in J·mol⁻¹); R corresponds to the ideal gas constant (R=8.314 J·K⁻¹·mol⁻¹); T corresponds to the temperature of the feedstock (in K).

In the embodiment for which stages a) and b) are carried out separately, i.e. the transformation of mercury to elemental mercury is carried out upstream of the separation unit 5000, the concentration C_(s) corresponds to the mercury concentration (apart from elemental mercury) measured in the line 102 at the inlet of the separation unit 5000, and the concentration C_(o) corresponds to the mercury concentration (apart from elemental mercury) measured in the line 101.

In the embodiment for which stages a) and b) are carried out simultaneously, i.e. the transformation of mercury to elemental mercury is carried out both during the transport of said feedstock to the separation unit 5000 and during the stage of separation of said feedstock in the separation unit 5000, the concentration C_(s) corresponds to the mercury concentration (apart from elemental mercury) measured in the line 203, and the concentration C_(o) corresponds to the mercury concentration (apart from elemental mercury) measured in the line 101.

Furthermore, according to the invention, the total volume V of the conversion unit 900 is defined such that the ratio V/Q, with Q corresponding to the volume flow of the feedstock to be treated, is equal to the residence time “t” associated with the targeted temperature of the feedstock “T”.

Thus, in the embodiment for which stages a) and b) are carried out separately, i.e. when the transformation of mercury to elemental mercury is carried out upstream of the separation unit 5000, the volume V of the conversion unit 900 corresponds to the volume of the heating unit 2000, such as a drum, and optionally to the pipe or the set of pipes intended for the transport of the feedstock to the separation unit 5000.

In the embodiment for which stages a) and b) are carried out simultaneously, i.e. when the transformation of mercury to elemental mercury is carried out both during the transport of said feedstock to the separation unit 5000 and during the stage of separation of said feedstock in the separation unit 5000, the volume V of the conversion unit 900 corresponds to the volume of the heating unit 2000, such as a drum, and optionally to the volume of the pipe or the set of pipes intended for the transport of the feedstock to the separation unit 5000, as well as to the volume of the separation unit 5000, in which unit the transformation of the non-elemental mercury contained in the compounds of the heavy hydrocarbon-containing feedstock to elemental mercury is also carried out.

With reference to FIGS. 2 and 3, the volume V of the conversion unit 900 corresponds to the cumulative volume of the heating unit 2000, of the pipe 102 and the volume of the separation unit 5000.

Advantageously, during the transformation stage, and according to any one of the embodiments according to the invention (i.e. stages a) and b) being carried out separately or not):

-   -   when the target temperature of said feedstock is comprised         between 150° C. and 175° C., the residence time of said         feedstock in the conversion unit 900 is comprised between 150         and 2700 minutes; and/or     -   when the target temperature of said feedstock is greater than         175° C. and less than or equal to 250° C., the residence time of         said feedstock in the conversion unit 900 is comprised between         100 and 900 minutes; and/or     -   when the target temperature of said feedstock is greater than         250° C. and less than or equal to 400° C., the residence time of         said feedstock in the conversion unit 900 is comprised between 5         and 70 minutes; and/or     -   when the target temperature of said feedstock is greater than         400° C., the residence time of said feedstock in the conversion         unit 900 is comprised between 1 and 10 minutes.

Even more preferably:

-   -   when the target temperature of said feedstock is comprised         between 150° C. and 175° C., the residence time of said         feedstock in the conversion unit 900 is comprised between 150         and 2700 minutes; and/or     -   when the target temperature of said feedstock is greater than         175° C. and less than or equal to 200° C., the residence time of         said feedstock in the conversion unit 900 is comprised between         100 and 900 minutes; and/or     -   when the target temperature of said feedstock is greater than         200° C. and less than or equal to 225° C., the residence time of         said feedstock in the conversion unit 900 is comprised between         30 and 300 minutes; and/or     -   when the target temperature of said feedstock is greater than         225° C. and less than or equal to 250° C., the residence time of         said feedstock in the conversion unit 900 is comprised between         15 and 150 minutes; and/or     -   when the target temperature of said feedstock is greater than         250° C. and less than or equal to 300° C., the residence time of         said feedstock in the conversion unit 900 is comprised between 5         and 70 minutes; and/or     -   when the target temperature of said feedstock is greater than         300° C. and less than or equal to 400° C., the residence time of         said feedstock in the conversion unit 900 is comprised between 1         and 40 minutes; and/or     -   when the target temperature of said feedstock is greater than         400° C. and less than or equal to 500° C., the residence time of         said feedstock in the conversion unit 900 is comprised between 1         and 10 minutes; and/or     -   when the target temperature of said feedstock is greater than         500° C., the residence time of said feedstock in the conversion         unit 900 is comprised between 1 and 5 minutes.

According to the invention, the stage of transformation of the non-elemental mercury contained in the feedstock to elemental mercury is carried out at a pressure comprised between 0.1 and 12 MPa, preferably between 0.1 and 6 MPa.

Thus it is possible to transform the non-elemental mercury contained in the compounds of said feedstock to elemental mercury, starting from 150° C., by adjusting the residence time of the feedstock in the conversion unit 900. Furthermore, the absence of a catalyst simplifies the implementation of the process and makes it possible to avoid the occurrence of clogging of the capture materials of heavy metals during the stage of bringing into contact said feedstock containing heavy compounds, likely to agglomerate or precipitate, with a mercury capture material.

b) Separation Stage

According to the invention, a stage of separation of the feedstock is carried out in a separation unit 5000, in order to produce a liquid effluent 103 and a gaseous effluent 203 comprising elemental mercury. The separation stage can be carried out by stripping (c.f. FIG. 2) or by distillation (c.f. FIG. 3). Preferably, a separation of the feedstock is carried out in a separation unit 5000 that consists of producing only a liquid effluent 103 and a gaseous effluent 203 comprising elemental mercury.

According to the embodiment shown in FIG. 2, i.e. when the separation unit 5000 is a stripping column, the feedstock originating from the heating unit 2000 is directed via the pipe 102 to a separation unit 5000 intended to separate said feedstock in order to produce a liquid effluent and a gaseous effluent. More particularly, the heavy hydrocarbon-containing feedstock sent to the separation unit 5000 circulates in counter-current with respect to the carrier gas sent to said separation unit via the pipe 202. The function of the separation unit 5000 is to eliminate the most volatile compounds from the feedstock, and more particularly elemental mercury.

The separation unit 5000 can comprise internals intended to promote exchanges between the liquid and vapour phases, such as plates or packing.

At the outlet of this separation stage, a liquid effluent 103 comprising a hydrocarbon-containing feedstock with reduced volatile fractions and mercury, and a gaseous effluent 203 are obtained.

Advantageously, the carrier gas used in the separation unit 5000 is a gas that is present at the operating site and the composition thereof is selected so as to not impact on the operation of the installations situated downstream. For example, the carrier gas is a cut of the hydrocarbon-containing compounds originating from the main fractionation unit 3000, and more particularly is a cut of the hydrocarbon-containing compounds of which 90% by weight of the compounds have a boiling point less than 200° C. at atmospheric pressure (1.01325×10⁵ Pa).

FIG. 3 shows another embodiment according to the invention in which the separation unit 5000 used for the separation stage b) is a distillation column. In this embodiment, the heavy hydrocarbon-containing feedstock, and more particularly a crude oil feedstock, is sent via the pipe 100 to a desalting unit 1000. The desalted feedstock is then sent via the pipe 101 to a heating unit 2000. The feedstock is then conveyed via the pipe 102 to the distillation column 5000. The mercury in elemental form and the most volatile hydrocarbon-containing compounds are recovered via the pipe 203 at the top of the distillation column. Typically, at the top of the distillation column 5000 a cut of the hydrocarbon-containing compounds is recovered of which 90% by weight of the compounds have a boiling point less than 200° C. at atmospheric pressure (1.01325×10⁵ Pa). In the distillation column, the internal gas and liquid traffic of the column will introduce in-situ stripping of the volatile compounds comprising mercury. Finally, the result is a top effluent recovered via the pipe 203 comprising at least 90% by weight of mercury with respect to the total weight of mercury present in the initial feedstock, sent to the distillation column 5000 via the pipe 102, preferentially at least 95% by weight and even more preferentially at least 99% by weight, and a bottom effluent recovered via the pipe 103 constituted by the initial feedstock with its light fraction and mercury-containing species reduced. The bottom flow recovered via the pipe 103 is then conveyed to a main fractionation unit 3000.

c) Stage for the Capture of the Mercury

The gaseous effluent recovered via the pipe 203, comprising the mercury in elemental form, is then sent to a, preferably single, unit for the capture of mercury 6000 comprising at least one capture material. The unit for the capture of mercury 6000 can be presented for example in the form of a fixed bed comprising a capture material containing an active phase suitable for reacting with the elemental mercury in order to immobilize it in the bed so as to produce a de-mercurized gaseous effluent which is introduced via the pipe 204 to the main fractionation unit 3000. Examples of effective capture materials are described in the patents FR 2764214, FR 2980722, or also FR 2992233.

The unit for the capture of mercury 6000 can also comprise means for adjusting the pressure and the temperature (not shown in the figures) in order to be adapted to the chosen method for the elimination of mercury.

Thus, unlike the state of the art shown in FIG. 1, the process according to the invention requires only one single type of mercury capture material, and one single unit for the capture of mercury, it being possible optionally to double said capture unit in parallel or in series in order to provide for maintenance without impacting on the operation of the fractionation unit. Furthermore, the process according to the invention makes it possible to recover the mercury contained in the compounds of the feedstock, and more particularly in the crude oil feedstock, upstream of a main fractionation unit in a refinery layout. Within the meaning of the invention, by refinery is meant the set of operations that make it possible to transform crude oil to oil products in current use. The crude oils are in the form of liquids that are more or less viscous essentially constituted by hydrocarbons of varying volatility and chemical composition.

d) Fractionation Stage

The liquid effluent recovered at the bottom of the separation unit 5000 is then sent via the pipe 103 to the main fractionation unit 3000. According to the invention, by main fractionation unit 3000 is meant a unit for the fractionation of the feedstock by atmospheric distillation (such as described previously in the section describing the prior art). The main fractionation unit 3000 can comprise one or more distillation columns (in FIG. 2, a single distillation column is shown). The main fractionation unit 3000 makes it possible to produce different hydrocarbon cuts depending on their molecular weight and more particularly depending on their difference in volatility.

In the embodiment as shown in FIG. 2, i.e. when the separation unit is a stripping column, a gaseous or liquid effluent is advantageously recovered via the pipe 200 at the top of the main fractionation unit 3000. When the effluent recovered at the top of the main fractionation unit is gaseous, it can originate directly from the gaseous fraction at the top of the main fractionation unit 3000. When the effluent recovered is liquid, it can originate from an intermediate liquid draw off from a plate of said column, preferably at the top of said column. Thus, the effluent passing through the pipe 200 can be in either gaseous or liquid form. The effluent recovered via the pipe 200 then passes through a pressurization means 4000 which can be in the form of a pump when the effluent passing through the pipe 200 is liquid, or in the form of a compressor when the effluent passing through the pipe 200 is gaseous.

Optionally, the effluent originating from the pressurization means 4000 is conveyed via the pipe 201 into a heat exchanger 2001. This stage is necessary when the effluent passing through the pipe 200 is in liquid form as it makes it possible to convert said effluent to gaseous form. Therefore, regardless of the nature of the effluent recovered via the pipe 200, the effluent passing through the pipe 202 is in gaseous form and is sent to the separation unit 5000 as a carrier gas (stripping gas).

In the separation unit 5000, the bringing into contact of the feedstock originating from the pipe 102 and the carrier gas originating from the pipe 202 makes it possible to recover, at the bottom of the separation unit 5000 via the pipe 103, a feedstock the most volatile fractions of which are reduced, and to recover at the top of the separation unit 5000 via the pipe 203, a gaseous stripping effluent mainly comprising the carrier gas, a light hydrocarbon fraction originating from the feedstock 102, more particularly light hydrocarbons from methane to octane, as well as mercury in elemental form in the gaseous state and optionally H₂S, entrained in the separation unit 5000 by the carrier gas.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 15/57.034, filed Jul. 24, 2016 are incorporated by reference herein.

Examples

The example below is based on the process according to the invention as shown in FIG. 2, i.e. the separation unit 5000 is a stripping column. The hydrocarbon-containing feedstock used is a crude oil feedstock originating from South-East Asia. In this example, it is assumed that the conversion unit 900 is composed of the heating unit 2000, the pipe 102 and the separation unit 5000.

After passing through a desalting unit 1000, the feedstock is sent to a conversion unit 900 in order to convert the non-elemental mercury contained in the compounds of said feedstock to elemental mercury. During this stage, the feedstock is heated to a target temperature of 180° C. (set by the operator) and during a fixed residence time adapted to the target temperature in order to allow the total conversion of the non-elemental mercury contained in said feedstock to elemental mercury.

Within the context of the present example, two tests were carried out by varying the residence time of the feedstock in the conversion unit 900. A first test was carried out by heating the feedstock to a target temperature of 180° C. for a residence time (contact time) of 60 minutes, and a second test was carried out by heating the feedstock to 180° C. for a residence time of 20 minutes (c.f. Table 1 below).

The separation unit 5000 is in the form of a liquid gas contactor, making it possible to carry out stripping said feedstock upstream of the main fractionation unit 3000. The flow of feedstock sent to the separation unit 5000 via the pipe 102 is 244 t/h of crude at a temperature of 380° C. and at a pressure of 1.5 MPa. The total flow of mercury contained in the feedstock is 45.5 g/h.

The feedstock sent via the pipe 102 is introduced at the top of the separation unit 5000. A carrier gas is introduced via the pipe 202 at the bottom of the separation unit 5000. The carrier gas will travel through the separation unit 5000 by rising to the top of the separation unit, carrying with it the most volatile compounds including the elemental mercury. The mainly liquid feedstock circulates in counter-current with the carrier gas. As a result, a gaseous stripping effluent is recovered at the top of the separation unit at the level of the pipe 203 at 371° C., 0.7 MPa and 101 t/h. The gaseous stripping effluent comprises mercury in elemental form.

The de-mercurized liquid feedstock is itself recovered below the separation unit 5000 via the pipe 103 at 1.4 MPa, 371° C. and 167 t/h and is conveyed to the main fractionation unit 3000. The main fractionation unit 3000 is here in the form of a distillation column with 34 theoretical plates operating at a pressure of between 0.43 MPa at the bottom and 0.39 MPa at the top for temperatures of 339 and 168° C. respectively.

A liquid effluent is extracted at the level of the liquid clearance of the top plate of the main fractionation unit 3000 via the pipe 200 that is pressurized by means of a pump 4000. The result is a liquid effluent passing through the pipe 201 at 1.5 MPa and 169° C. The liquid effluent is re-heated to 350° C. by a heat exchanger 2001 via the gaseous stripping effluent circulating in the pipe 203. The result is a gaseous effluent passing through the pipe 202 at 350° C. and 1.49 MPa.

The gaseous stripping effluent recovered at the top of the separation column 5000 via the pipe 203 (comprising mercury in elemental form) is sent to a unit for the treatment of mercury 6000 comprising a mercury capture material based on CuS deposited on alumina, suitable for the capture of mercury in elemental form. The result is a de-mercurized flow circulating in the pipe 204 at 0.6 MPa and 150° C. and which is redirected to the main fractionation unit 3000.

For the two tests carried out, the contents of total mercury and elemental mercury in the pipes 101, 102, 103, 203 and 204 are determined in μg/L.

Liquid samples are taken for the pipes 101, 102 and 103, then analysed using a specific PE-1000® device from Nippon Instruments Corporation (NIC) for mercury analyses. In order to determine the elemental mercury content, the effluent for analysis is analyzed in the PE-1000® at the same time and stripped of nitrogen so as to eliminate the elemental mercury. By analysing the mercury content of the sample after stripping and subtracting it from the mercury content of the sample before stripping, the quantity of elemental mercury initially present in the effluents is calculated.

In the pipes 203 and 204, the effluent is gaseous. The mercury is measured with a SP3D® device (NIC) adapted for gas analysis.

TABLE 1 Total and elemental mercury content in the different pipes Target conversion temperature and contact time 180° C./60 min 180° C./20 min Pipe 101 [Hg_(total)] 202 200 [Hg°] 0 0 Pipe 102 [Hg_(total)] 201 199 [Hg°] 197 136 Pipe 103 [Hg_(total)] 0 10 [Hg°] 0 9 Pipe 203 [Hg_(total)] 199 134 Pipe 204 [Hg_(total)] 0 1

The mercury content measured in the pipe 203 is identical to that of the elemental mercury content in the pipe 102. It is therefore only elemental mercury.

The mercury contents measured show that in order to be able to transform the non-elemental mercury contained in the compounds of the feedstock to elemental mercury, the contact time at a target temperature must be sufficient. If this is not the case or if the flow is not heated, decomposition of the refractory species comprising the mercury in the feedstock is not total and a part of the mercury is recovered at the bottom of the separation device. Consequently, the different units situated downstream in a refinery layout are therefore potentially polluted with mercury and it will be necessary to add as many capture materials as there are outlet flows, leading to a significant overspend in terms of investment and operating costs.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. Process for the elimination of mercury contained in a heavy hydrocarbon-containing feedstock upstream of a main fractionation unit (3000), a process in which: a) the non-elemental mercury contained in the compounds of said feedstock is transformed to elemental mercury, said stage being carried out in a conversion unit (900) at a target temperature during a fixed residence time and adapted to said target temperature so that at least 90% by weight of non-elemental mercury contained in the compounds of said feedstock is converted to elemental mercury, said transformation stage being carried out in the absence of hydrogen and in the absence of a catalyst, it being understood that: when the target temperature of said feedstock is comprised between 150° C. and 175° C., the residence time of said feedstock in the conversion unit (900) is comprised between 150 and 2700 minutes; and/or when the target temperature of said feedstock is greater than 175° C. and less than or equal to 250° C., the residence time of said feedstock in the conversion unit (900) is comprised between 100 and 900 minutes; and/or when the target temperature of said feedstock is greater than 250° C. and less than or equal to 400° C., the residence time of said feedstock in the conversion unit (900) is comprised between 5 and 70 minutes; and/or when the target temperature of said feedstock is greater than 400° C., the residence time of said feedstock in the conversion unit (900) is comprised between 1 and 10 minutes; b) a separation of the feedstock obtained in stage a) is carried out in a separation unit (5000), in order to produce a liquid effluent (103) and a gaseous effluent (203) comprising elemental mercury; c) the gaseous effluent (203) originating from stage b) comprising the elemental mercury is brought into contact with a mercury capture material contained in a unit for the capture of mercury (6000), in order to produce an effluent that is at least partially de-mercurized (204).
 2. Process according to claim 1, comprising moreover a stage d) in which the liquid effluent (103) obtained in stage b) is fractionated in a main fractionation unit (3000).
 3. Process according to claim 1, characterized in that the reduction in the total content by weight of mercury of said feedstock taken before stage a) and after stage c) is at least 90%.
 4. Process according to claim 1, characterized in that the stages a) and b) are carried out separately or simultaneously.
 5. Process according to claim 1, characterized in that the separation unit (5000) of stage b) is a distillation column.
 6. Process according to claim 1, characterized in that the separation unit (5000) of stage b) is a stripping column.
 7. Process according to claim 6, characterized in that in the stripping column a carrier gas circulates in counter-current with said hydrocarbon-containing feedstock, said carrier gas at least partially originating from a liquid or gaseous fraction of the main fractionation unit (3000).
 8. Process according to claim 7, in which when the carrier gas at least partially originates from a liquid fraction of the main fractionation unit (3000), said liquid fraction is transformed to a gaseous fraction by means of a heat exchanger (2001).
 9. Process according to claim 6, characterized in that the at least partially de-mercurized effluent (204) obtained in stage c) is fractionated in a main fractionation unit (3000).
 10. Process according to claim 1, characterized in that said hydrocarbon-containing feedstock comprises between 1 and 10 mg of mercury per kg of feedstock.
 11. Process according to claim 1, characterized in that during stage c), said feedstock is brought into contact with a bulk or supported mercury capture material comprising a phase containing at least one metallic sulphide based on a metal M selected from the group constituted by copper, chromium, manganese, iron, cobalt and nickel.
 12. Process according to claim 1, characterized in that during stage c), said feedstock is brought into contact with a bulk or supported capture material comprising a phase containing at least sulphur in elemental form.
 13. Process according to claim 1, characterized in that the heavy hydrocarbon-containing feedstock is a crude oil feedstock. 